Looks like the whole world is starting up home improvement projects now that spring appears to be here. Me too... First job. Would very much like to turn our garage into a much more usable space (insulate) - would obviously like to do this in the most eco-friendly manner possible (I work for a renewable energy company - been very unhelpful on insulation)...

All of the best currently available insulation products rely on air as the magic ingredient that slows the passage of heat in or out.

You cannot stop heat from escaping/arriving, only slow it down.

Polystyrene is one of the cheapest insulation's and the most effective inch for inch.

Polystyrene is 98% air and 2% plastic.

It can be sprayed in, to create a monolithic structure, that stops heat escaping/arriving through cracks and holes in a building.

You can load things onto polystyrene, it doesn't compress, it is water proof.

Cotton is not a good insulation, heat travels by conduction through the fabric, think of wearing jeans on a hot summers day.
Trying to bulk up cotton, to increase its air content fails in the medium term as the cotton fabric is fighting a loosing battle with gravity.
Eventually gravity wins, the fabric compresses and becomes useless.